Mendel-Brno 2000

category image Volume: 17
Issue Number 6, Part 2
June 2000

Probing DNA Conductivity by Photoinduced Electron Transfer and Scanning Probe Microscopy

The fluorescent DNA probe [Ru(phen)2(dppz)]2+ is the first ruthenium complex to be shown unequivocally to intercalate DNA by insertion of the dppz ligand. It has not been possible to obtain a structure of the bound complex by NMR, so we have combined polarized spectroscopy and photophysical techniques to characterize the binding by studying interactions with several nucleic acids. The non-intercalating ligands have been varied to investigate the possibility of homo-cooperative binding; modelling indicates favourable stacking interactions between these ancillary ligands when the complexes intercalate with nearest-neighbour exclusion.

There has been considerable speculation during the past few years about the possibility that DNA can act as a wire and mediate charge transport over long distances via the p-systems of the basepair stack. The photophysics of
*[Ru(phen)2(dppz)]2+ have been investigated with time-resolved fluorescence, transient absorption and time-resolved resonance Raman spectroscopy on the picosecond to nanosecond timescales. Photoinduced electron transfer reactions of *[Ru(phen)2(dppz)]2+ in the presence of DNA have been studied, to address whether DNA mediates charge transfer processes between non-covalently bound ligands or whether it acts simply as an anionic scaffold, concentrating the cationic reactants and forcing them into close proximity. Results suggest that externally bound acceptors quenches much more efficiently than intercalated acceptors and that electron transfer between intercalators separated by >18Å is unlikely. Quenching in DNA-bound donor-bridge-acceptor intercalator pairs has also been studied and indicates that fast electron transfer may occur between intercalators separated by two basepairs but with low efficiency.

In complementary studies we have investigated the conductivity of single DNA molecules immobilised in a self-assembled monolayer on gold using scanning tunneling microscopy and conducting atomic force

Eimer Tuite, Per Lincoln, Björn Önfelt, Johan Olofsson, Donats Erts*, Bengt Nordén

Department of Physical Chemistry, Chalmers University of Technology,
S-412 96 Göteborg, Sweden,
*Institute of Chemical Physics, University of Latvia, LV-1586 Riga, Latvia